The tablet market is different then the phone market. The reason android enjoyed so much success was the iPhone only being on one carrier for 4 years.
America is not the world. In most European countries the iPhone has been available on multiple carriers for a couple of years, and Android has still overtaken it.
But I think 3 years is probably about right. Android needs to build brand recognition in this space and right now Apple have a stranglehold on this market in terms of sales and consumer recognition in a way they never had in the phone market. For most consumers right now the iPad is the only tablet.
Beyond that, there's a problem for Android OEMs when it comes to actually competing with Apple in terms of both price and design. Most will be forced to make trade-offs in design terms in order to match or beat Apple's price (ie thicker, heavier devices like the Xoom and Transformer). There's also an issue with securing certain parts like screens and flash memory, where Apple's purchasing power means they can lock down supply for a year or two in advance.
As of right now only Samsung is in a position to genuinely fight that battle, since they control an entire production chain, from research and design to manufacturing.
That might change once the industry adjusts itself to fully exploit the tablet market but it won't happen quickly. For the next year I think it will be largely Samsung vs Apple, with the latter taking the lion's share of the sales, and HTC and others picking up the leftovers.
Of course the Amazon tablet could go a long way towards giving Android the upper hand in a shorter timeframe, but we'll have to wait and see what they come up with.
tl;dr - it will happen sooner or later but the tablet market is a different beast from the phone market.
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